Fluorocarbon polymers, such as, for example, polytetrafluoroethylene (PTFE), polymers of chlorotrifluoroethylene (CTFE), of hexafluoropropylene (HFP), fluorinated ethylene-propylene copolymers (FEP), and polyvinylidene fluoride (PVDF), are known to have superior non-stick properties. For this reason, they have been used in a wide variety of applications, including forming non-stick coatings on articles of cookware, bakeware, iron sole plates, food contacting surfaces of small appliances, snow shovels and plows, chutes and conveyors, saws, hoppers, and other industrial containers. However, due to the inherent non-stick nature of these and other fluorocarbon polymers, it has been difficult to form non-stick coatings that adhere well to substrates such as metals and ceramics. Moreover, due to the inherent softness of fluorocarbon polymers, it has been difficult to form non-stick coatings that resist abrasion.
In an effort to overcome these difficulties, it has been the conventional practice to apply one or more base coats containing adhesive resins in order to better adhere fluorocarbon polymer top coats to substrates (throughout this specification and in the claims, the terms “base coat” and “primer coat” are used interchangeably). In general, such base coats comprise a combination of high temperature binder resins, such as polyamideimide resins, polyethersulfone resins, or polyphenylene sulfide resins, and fluorocarbon polymer resins. The performance of these conventional non-stick coating systems is based upon a stratification of the applied coatings. This stratification results in a coating that is rich in high temperature binder on the bottom and rich in fluorocarbon polymer at the top. The binder-rich bottom provides adhesion to the substrate while the fluorocarbon polymer-rich top provides a layer to which subsequent fluorocarbon polymer top coats can be fused by sintering at high temperature.
The performance of such non-stick coating systems is at best a compromise. The bottom layer of the base coats is not a purely binder resin. Considerable levels of fluorocarbon polymer resins must be included in the base coats in order to provide a layer that is sufficiently rich in fluorocarbon polymer to promote good bonding of subsequent fluorocarbon polymer top coats to the base coat. The presence of fluorocarbon polymer resins in the base coat are disadvantageous because they detract from the adhesion of the base coat to the substrate. Therefore, it has been necessary to roughen substrates by mechanical (e.g. grit blasting) or chemical (e.g. etching) means to assist holding the base coat to the substrate.
Moreover, because both the adhesive resins and fluorocarbon polymers are relatively soft, there have been difficulties in making these non-stick coatings resistant to abrasive wear. Efforts to overcome these deficiencies have included the addition of mica particles, ceramic fillers, or metal flakes to the intermediate and top coat in order to increase the hardness. The presence of these fillers can be disadvantageous. For example, incorporation of metal flakes in the applied coatings can actually promote chemical corrosion of the underlying metal substrate due to dissimilarity between the metals. Moreover, these particulate fillers cannot be incorporated into the non-stick coating at high levels because at high levels they diminish the non-stick properties of the coating and the bonding to the substrate.
Due to the limitations thus described, articles of cookware coated with conventional fluorocarbon polymer non-stick coating systems are prone to damage and abrasive wear during normal use. Cooking utensils, for example, often cause cuts, slices, or gouges in the non-stick coating which permit acids or alkaline foodstuffs and cleaning agents to penetrate to the exposed substrate and cause corrosion. Corrosion of the underlying substrate by these materials can further weaken the adhesion of the non-stick coating adjacent to the cut or slice. Moreover, abrasive forces routinely encountered in cooking and cleaning cause the gradual removal of the soft fluorocarbon polymer top coat resulting in diminished non-stick properties. Conventional non-stick coatings simply do not adequately protect the substrate from corrosion or the fluorocarbon polymer top coat from routine abrasive wear.
Compositions are known for use in forming porcelain enamel coatings that are very resistant to abrasion and chemical wear. Unfortunately, such known porcelain enamel coatings do not possess non-stick properties comparable to fluorocarbon polymer coatings. Moreover, such known porcelain enamel coatings are generally not considered to possess so called “Easy-to-Clean” properties. Despite considerable effort, past attempts to formulate a composition that can be used to form a coating on substrates that exhibits the excellent non-stick attributes of fluorocarbon polymer coatings as well as the abrasion resistance of porcelain enamels have heretofore been largely unsuccessful.